Casting magnesium



. hand, it the amount fromoxidation. Various these substances are likewise not altogether Patented Sept. 21, 1943 William R. Caple, Evanston; 111.. and

Throne, Midland, Mlehmsslgnors to Richard I. The Dow chemical Company,Mldland,Mich., a corporation of Michigan No Drawing. 4 Application August 1, 1942,

Serial No.45a,24s

invention relates to improvements in the castingof magnesium and magnesium-base alloys in temporary molds. a The production or commercially acceptable castings of magnesium-and magnesium alloys presentsa problem which is unique in foundry practice duefto the aflinity for both oxygen and-nitrogen which this metal possesses, especially at elevated temperatures; In the molten state, at thetemperatures employed in casting, the metal is capable of combining spontaneously with oxygen, which maybe derived either from the air or irom substances containing free or combined oxygen which are in contact with the metal, such as water, steam, or other compounds present in the material constituting the casting mold. This burning or oxidation of molten magnesium may at times be so violent as toapproach explosiveness. Thus, it is practically impossible to make sound castings otmagnesium in simple greensand molds,such as are used forv casting other metals.-

Because of this problem, it is conventional to incorporate in the material forming the mold some addition agent for overcoming'the tendency of water or other oxygen compounds in the mold to attack magnesium when the latter is cast therein. For example, it has been su gested to sulphur or boric acid into a water-bound sand mold for casting magnesium to overcome harmful efiectsof air or water vapor on the metal. However, these substances have proved not to be wholly. satisfactory, inasmuch as when a'suflicient amount of either sulphur or boric acid is added to themolding sand to prevent oxidation of a casting or relatively heavy section, thin sections are liable to be pitted. n the other or agent is reduced to a thin sections is minimized, be insufllciently protected other protective agents such as simple and complex fluorides have also been suggested-for preventing; theburning 0!. magnesium when cast in waterbound molds. but

point where pitting'oi heavier sections may are lost rapidly during cause the sand to swell. or the mold.- ln 'addisatisiactory, since they use and since they may with resulting distortion tion, the use of fluorides is somewhat hazardous because 01 the toxicity of these materials..

In view or these difliculties, it is the principal obiect of the present invention to provide a method of casting magnesium and magnesiumbase alloys in temporary molds in which there are utilized addition agents which eflectively 5 oxidation or burning of magnesium and mag-.

I be, satisfactorily overcome Y without being protect the metal against oxidation during casting, but which are not subject to the disadvantages of agents heretofore known.

According to the invention, the tendency for nesium alloys when cast in temporary molds may the molding material prior to the casting of the magnesium a small proportion of an aromatic ester of a phosphorus acid. "These new agents permit making castings or either relatively heavy or light section substantially without'burning,

pitting, or the formation or blow holes or'similar fiaws. They are less volatile under casting conditions than those heretofore used, so that their effectiveness is lost very slowly during repeated use of the sand. At the same time they'dp not have a'detrimental efiect on the molding quality of foundry sands, butv actually render it workable for a longer period oI-time and lower the dry strengthoi the sand, making later shakeout much easier. In addition, the new agents and their thermal decomposition products are nontoxic and accordingly do not hazard.v M

In so far as is known,,any aromatic ester of a phosphorus acid may be employed as the addition agent or the invention, However, the aromatic tri-esters of phosphoric acid are preierred, especially the triaryl vphosphates, such as triphenyl phosphate, tricresyl phosphate, diphenyl-mono-ortho'xenyl .phosphate, tri-(paratertiarybutyl phenyl) phosphate, di-(para-tere tiary-butyl phenyl) orthoxenyl-mono-phenyl-phosphate, and (4-tertiary-butylphenyl) phenyl ortho-xenyl phosphate. However, other aromatic esters of phos-v phorus acids. such as the tri-haloaryl phosphates, e. g. tri-(2.4.6-tri-chlorophenyl)' phosphate, aromatic'tri-esters of phosphorous acid, e. g.- di-' (2-xenyl) phenyl' phosphite, certain esters of such as phenyldi=(metatolyl) thiophosphate and di-phenyl' ortho-xenyl thiophosphate, may alsdbe used, Those esters which are liquidate ordinarily preferred because 01 their ease or application. Esters boiling at an absolute pressure or 5 millimeters or men cury in the range or 250 to 300 C. are most satisiacwmsince they during use,

The amount or thee new agent which may be employedvaries with the size and type of casting 5 .tobemadeaswell as by incorporating in constitute a health monophenyl phosphate, di-

7 are sufllciently volatile to aflord adequate protection to the so volatile as .to be lost rapidly Q with nature or the mold-i lug material, which is usually foundry sand. In making large castings a greater proportion of the agent may be required than when relatively small castings are made. Normally, foundry molding material may be intimately mixed with from 0.5 to 10 per cent by weight of any one of the compounds previously mentioned, although usually about 1 to 5 per cent is satisfactory. Mixtures of two or more of these substances may be employed, the sum of which preferably does not exceed about 10 per cent. It is understood, 7

however, that the quantity of one or more of such agents in any given case is a matter of judgment, governed more or less by the-size and character of the casting.

In the casting of magnesium and its alloys according to the inventiomthe preparation of the mold and cores is carried out entirely in accordance with known practice, except that the phosphorus acid esters are used in place of the addition agents heretofore employed. In generaL'in the case of green-sand castings, the new agent is incorporated into the water-tempered foundry sand or synthetic sand, such as a silica-bentonite mixture, by mechanical mixing, after which the sand is formed into molds and cores, the latter with addition of a binder, in conventional manner. Alternatively, mold facings may be prepared from foundry sand mixed with the new agents,

and the mixture applied to the pattern in the usual fashion. Again, the new agents may be dissolved in a volatile solvent, such as carbon tetrachloride, and the solution'then sprayed on the mold parts which come in contact with the molten magnesium, or these partsmay be dusted with a powdered solid triaryl phosphate.

The use of the new agents, furthermore, is not necessarily limited to molds made from sand,

since other moldable materialssuch as graphite, pulverized carbon, or the like may be substituted for the sand. The agents are also useful in connection with semi-permanent molds, such as those made of plaster.

The following example will serve to illustratethe invention, but is not to be construed as limiting its scope.

Example A water-tempered foundry sand having an' A. F. A. permeability factor of about 80 was mulled together with 5 per cent by weight of di- (para-tertiary-butylphenyl) monophenyl phosphate, and the mixture was formed into a mold.

A casting was then prepared by pouring a quantity oi Dowmetal H (a magnesium-base alloy containing 6.0 per cent aluminum, 3.0 per cent zinc and "0.2 per cent manganese, the balance being magnesium) into the mold, the pouring aromatic ester of a phosphorus acid into the mold material prior to the casting of the metal.

2. In a process of casting magnesium and magnesium-base alloys in' green-sand molds, the improvement which comprises incorporating .between 0.5 and 10 per cent by weight of an aromatic tri-ester of phosphoric acid into the sand prior to the casting of the metal,

' 3. A process according to claim 2 wherein the ester employed is a normally liquid tri-aryl phosphate. v p

4. The method of casting magnesium and magnesium-base alloys in temporary molds which comprises incorporating into the molding material an aromatic ester of a phosphorus acid, forming a mold of the resulting mixture, and casting the metal therein.

5. In a process of, casting magnesium and magnesium-base alloys in sand molds, the method of minimizing oxidation .of \the metal during casting which comprises pouring the, molten metal into a mold in which is incorporated, at

least inthe' portions adjacent the molten metal,

a small proportion of an aromatic tri-ester of phosphoric acid.

6. A temporary mold for casting magnesium and magnesiumhasa alloys having mixed with the molding material a small proportion of an aromatic tri-ester of a phosphorus acid.

7. A sand mold for casting magnesium and magnesium-base alloys having mixed with the sand between 0.5 and 10 per cent by weight of a triaryl phosphate.

WILLIAM R. CAPLE. RICHARlJ I. THRUNE. 

